Common rail for diesel engines

ABSTRACT

A common rail for a diesel engine has a main pipe rail with an axial flow passage. Branch holes are made in the main pipe rail and communicate with the axial flow passage. The main pipe rail is worked to decrease stress concentration near the branch holes. The whole inner peripheral surface of the main pipe rail and the branch holes then is subjected to autofrettage processing to generate a compressive residual stress on the entire inner peripheral surface of the main pipe rail and the branch holes for increasing fatigue strength.

This application is a divisional U.S. patent application Ser. No. 10/653,477, filed Sep. 2, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a common rail, such as high-pressure fuel manifolds, block rails, or the like, in a fuel injection system for diesel engines, and more particularly, to a common rail for diesel engines, in which portions of a branch hole or holes on a main pipe rail and a whole inner peripheral surface of the main pipe rail are enhanced in fatigue strength for inner pressures.

2. Description of the Related Art

A common rail in a fuel injection system for diesel engines is shown in FIG. 7, and has a circular main pipe 11. The main pipe 11 has an internal flow passage 11-1. A branch hole 11-2 is provided in a peripheral wall of the main pipe 11 and communicates to the internal flow passage 11-1. The branch hole 11-2 leads to an outwardly opened pressure-receiving seat surface 11-3. A cylindrical-shaped sleeve nipple 13 is mounted on an outer peripheral wall of the main pipe 11 in the vicinity of the pressure-receiving seat surface, as by welding or brazing. A pressing seat surface 12-3 of a connection head 12-2 on a branch pipe 12 is caused to abut against and engage with the pressure-receiving seat surface 11-3 of the main pipe 11, and connection is achieved by clamping. More particularly, a clamping nut 14 is assembled beforehand to the branch pipe 12 and is screwed into the nipple 13 to apply pressure below the connection head 12-2. In the figure, the reference numeral 12-1 denotes a flow passage in the branch pipe 12.

However, the common rail of this kind involves the possibility that a large stress is generated at an inner peripheral edge P of a lower end of the branch hole 11-2 due to inner pressure in the main pipe 11 and axial forces applied to the pressure-receiving seat surface 11-3 due to pressure applied by the connection head 12-2 of the branch pipe 12. Thus, cracks are liable to generate with the inner peripheral edge P as a starting point and can cause leakage.

The inventor of the present application has proposed, as a countermeasure for the above, a common rail, in which a maximum stress value generated at an inner peripheral edge P of a lower end of a branch hole is decreased and fatigue strength for inner pressures is enhanced.

For example, a first proposal for a common rail increased in fatigue strength for inner pressures uses a pressing mold to flatten the inner peripheral edge of the lower end of the branch hole for decreasing stress concentration generated at the inner peripheral edge of the lower end of the branch hole. A second proposal provides a common rail that is increased in fatigue strength for inner pressures by creating a compressive residual stress in and around an end of the branch hole that is opened to the flow passage of the main pipe. The compressive residual stress cancels a tensile stress generated at the inner peripheral edge of the lower end of the branch hole by high inner pressures in the main pipe (see JP-A-10-306757, JP-A-10-318081, JP-A-10-318082, JP-A-10-318083, and so on). The method of making compressive residual stresses present in and around an end of the branch hole opened to a flow passage in the main pipe can include: applying pressing forces by means of a press system or the like; applying pressure in a flow passage in a main pipe; a pipe expanding method of mechanically applying pressing forces from an interior of a main pipe in a diametrical direction of the pipe, a diameter expanding method of mechanically applying pressing forces from an interior of a branch pipe in a diametrical direction of the pipe, or the like. A third proposal provides a common rail increased in fatigue strength for inner pressures, in which stress concentration generated at an inner peripheral edge of a lower end of a branch hole is decreased by inserting a branch pipe or a branch joint deeply into a branch hole, and fixing the pipe or joint with a tip end of the pipe or joint projecting into an interior of a flow passage from an inner peripheral wall surface of a main pipe rail (Japanese Patent Application No. 2001-387366). A fourth proposal provides a common rail increased in fatigue strength for inner pressures, in which stress concentration generated at an inner peripheral edge of a lower end of a branch hole is decreased by providing a flattened surface on an end of a branch hole opened to a flow passage of a main pipe rail and fixing the branch pipe so that the branch pipe projects into the flow passage from the flattened surface (Japanese Patent Application No. 2001-11772).

However, the common rails previously proposed by the inventor of the present application involve the following matters that can be improved upon.

More specifically, (the first and second proposals described above have an effect on the flattened portion or a portion in which stress is residual, but do not have any adequate effect on other portions, such as the flow passage of the main pipe or a flow passage of the branch hole. Also, the third and fourth proposals described above decrease stress concentration at corner portions of the flow passage in the branch hole. However, degradation in fatigue strength for inner pressures is unavoidable since brazing is necessary to carry out the third and fourth porposals and portions applied by such thermal influences are necessarily degraded in strength.

A set pressure for common rails in a fuel system for diesel engines has been conventionally around 135 Mpa. However, pressure as high as 160 Mpa presently tends to be the main stream and common rails for 180 Mpa are being developed for the future. Correspondingly, strength required for common rails must be able to include pulsation and to conform to +20 Mpa, and it is expected that common rails usable at 200 Mpa (180 Mpa+20 Mpa) will be needed in the near future.

The invention has been thought of to solve the above-mentioned problems and to accommodate systems with a high set pressure of around 200 Mpa. Thus, an object of the invention is to provide a common rail, in which a flow passage in a main pipe rail and portions of a branch hole are improved in durability while the portions of the branch hole are enhanced in fatigue strength for inner pressures.

SUMMARY OF THE INVENTION

A common rail for diesel engines according to the invention, in which a branch hole is made in a main pipe rail, having therein a flow passage in an axial direction. A branch hole is made in the main pipe rail and communicates to the flow passage. Stress concentration in the vicinity of inner peripheral edges of openings of the branch holes is decreased, and then all inner peripheral surfaces of the main pipe rail and the branch holes are subjected to autofrettage processing. Alternatively, at least neighborhoods of openings of the branch holes may be flattened to decrease stress concentration, and then all of the inner peripheral surface of the main pipe rail and the branch holes are subjected to autofrettage processing. In a further embodiment, pressing forces are applied to neighborhoods of ends of the branch holes opened to the main pipe rail to leave compressive residual stresses in the neighborhoods of openings of the branch holes for decreasing stress concentration, and then all inner peripheral surface of the main pipe rail and the branch holes are subjected to autofrettage processing. In another embodiment, stress concentration in the vicinity of the branch holes is relaxed by inserting tip ends of branch pipes deeply into the branch holes beyond an inner peripheral surface of the main pipe rail up to an interior of the flow passage and firmly connecting the branch pipes to the main pipe rail. Then the entire inner peripheral surface of the main pipe rail and interiors of connections of the branch holes are subjected to autofrettage processing. Here, autofrettage processing means application of stresses by exerting pressing forces on a pipe by means of an internal pressure system.

It is well known that when the autofrettage processing is applied on a thick-wall pipe, compressive residual stresses are generated and an improvement in fatigue strength for inner pressures is achieved. In common rails for diesel engines, however, disadvantages are caused when autofrettage is applied without application of further processings in the stock pipe as described herein.

For example, consider the case where a branch hole of 0.3 mm is made in a main pipe that has an outside diameter of 24 mm, an inside diameter of 7 mm, a pipe tensile strength Ts=650 Mpa, and a yielding point Yp=450 Mpa. In this situation, a circumferential stress on an inner surface amounts to 240 Mpa upon application of internal pressure of 100 Mpa. Furthermore, a maximum tensile stress at an opening peripheral edge of the branch hole in a circumferential direction of the inner surface results in stress concentration of 531 Mpa (2.6 times 240 Mpa).

Meanwhile, in the case where pressure at which the autofrettage processing is applied amounts to pressure, at which a portion of the main pipe as far as a center of wall thickness is subjected to plastic deformation, an internal pressure of 350 Mpa is needed according to a stress calculating formula (Tresca's formula). When the pressure of the autofrettage processing is applied, a location of stress concentration at the opening peripheral edge of the branch hole is subjected to 2.6 times the pressure on other portions. More specifically, it can be readily estimated that the location of stress concentration at the opening peripheral edge of the branch hole is put in the same state as that when pressure of 910 Mpa is applied, that is, 2.6 times 240 Mpa, and suffers cracks.

On the other hand, in the case where pressure is selected to leave compressive residual stresses at the location of stress concentration, a stress generated becomes 2.6 times 350 Mpa, so that a value obtained by dividing 350 Mpa by 2.6 can be selected, that is, 134.6 Mpa. It is shown that the value is lower than a demanded working pressure 160 Mpa and use is not possible at a pressure above 134.6 Mpa.

As can be deduced from the above description, pressure, at which a suitable residual stress is obtained, is not present in either the case where the flow passage in the main pipe is aimed at, or the case where the location of stress concentration is aimed at. From the above, there is a need of decreasing stress concentration in order that autofrettage is used to leave compressive residual stresses on inner surfaces of all flow passages in the main pipe and the branch hole.

According to the invention, since the processing of any one of the four proposals described above or a combination thereof is carried out, and then autofrettage pressure of, for example, 350 Mpa is applied, compressive stresses of about 399 Mpa are left in the flow passage in the main pipe (according to Tresca's formula) Meanwhile, since stresses are not concentrated or relaxed at the portion of the branch hole, residual stresses of 99 Mpa or there around can be expected.

As described above, stress concentration at the peripheral edge of the branch hole is relaxed and autofrettage is then applied according to the invention. Thus, high compressive residual stresses can be made present in the whole common rail due to the synergistic effect of a decrease of stress concentration applied at the peripheral edge of the branch hole and autofrettage applied on the whole flow passage in the main pipe rail and the flow passage in the branch hole. Accordingly excellent durability can be expected also for the flow passage in the main pipe rail and the opening peripheral edge of the branch hole while fatigue strength for inner pressures is increased at the inner peripheral edge of the lower end of the branch hole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially enlarged, cross sectional view showing a first embodiment of a common rail for diesel engines, according to the invention;

FIG. 2 is a longitudinal, cross sectional view of FIG. 1;

FIG. 3 is a partially enlarged, cross sectional view showing a second embodiment of the invention;

FIG. 4 is a longitudinal, cross sectional view of FIG. 3;

FIG. 5 is a partially enlarged, cross sectional view showing a third embodiment of the invention;

FIG. 6 is a partially enlarged, cross sectional view showing a fourth embodiment of the invention;

FIG. 7 is a longitudinal, cross sectional view showing a first example of a conventional common rail for diesel engines;

FIG. 8 is a longitudinal, cross sectional view showing a second example of a conventional common rail for diesel engines;

FIG. 9 is a longitudinal, cross sectional view showing a third example of a conventional common rail for diesel engines;

FIG. 10 is a longitudinal, cross sectional view showing a fourth example of a conventional common rail for diesel engines; and

FIG. 11 is a longitudinal, cross sectional view showing a fifth example of a conventional common rail for diesel engines.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the invention, reference numeral 1 denotes a main pipe rail, 1-1 a flow passage, 1-2 a branch hole, 1-3 a pressure-receiving seat surface, 1-4 a boss portion, 1-4 a a male thread, 1-5 a flattened surface, 2 a branch pipe, 2-1 a flow passage, and 2-2 a projection.

The main pipe rail 1 or common rail shown in FIGS. 1 to 4 is a forging of a material S45C or the like. The main pipe rail 1 has a tubular portion, which is relatively thick-walled to have, for example, a diameter of 28 mm and a wall thickness 9 mm. An axial interior is subjected to machining, such as boring, gun drilling, or the like, to make the flow passage 1-1, and boss portions 1-4 are provided on a peripheral wall to be axially spaced from one another.

In the common rail shown in FIGS. 1 and 2, branch holes 1-2 having a predetermined diameter are formed in the boss portions 1-4 which are integral with the main pipe rail 1 The branch holes 1-2 communicate with the flow passage 1-1 of the main pipe rail 1. Conical outwardly open pressure-receiving seat surfaces 1-3 are formed at the outward opening ends of the branch holes 1-2, and male threads 1-4 a are worked on outer peripheries of the boss portions 1-4. The pressing method or the like is used to form flattened surfaces 1-5 on an inner peripheral surface of the main pipe rail 1 toward the branch holes 1-2, preferably over the whole axial length, and on at least peripheral edges of openings of the branch holes 1-2. Thus stress concentration generated at inner peripheral edges of lower ends of the branch holes 1-2 is decreased to increase inner pressure strength at the inner peripheral edges of the lower ends of the branch holes. Autofrettage processing then is applied to generate compressive residual stresses on the whole inner peripheral surface of the main pipe rail 1 and all of the branch holes 1-2, thereby increasing the fatigue strength for inner pressures.

The flattened surfaces 1-5 can be formed by several methods, such as: applying pressing forces by means of, for example, an external pressure system to form flattened surfaces on an inner peripheral wall surface, forming flattened surfaces on an inner peripheral wall surface at the time of forging, forming flattened surfaces at the time of extrusion molding, and so on. Incidentally, with the method of a applying pressing forces by means of an external pressure system to form flattened surfaces on an inner peripheral wall surface, the flattened surfaces include, in some cases inwardly projecting arcuate surfaces. Accordingly, the flattened surfaces referred to in the invention are not completely flat surfaces, but include various curved configurations such as the arcuate surfaces, elliptical-shaped surfaces, or the like.

Also, the autofrettage processing is a process in which pressing forces produced by a fluid pressure are applied to the flow passage in the main pipe rail 1 to exert stresses on the whole inner peripheral surface of the main pipe rail 1 and in the branch holes 1-2.

The common rail shown in FIGS. 3 and 4, has branch holes 1-2 of a predetermined diameter formed in boss portions 1-4 that are integral with a main pipe rail 1 so that the branch holes communicate with a flow passage 1-1 of the main pipe rail. Conical outwardly open pressure-receiving seat surfaces 1-3 are formed at the outward opening ends of the branch holes 1-2 and female threads 1-4 b are worked on inner peripheries of the boss portions 1-4. The pressing method or the like is used to project a side of an inner peripheral surface of the main pipe rail 1 toward the branch holes 1-2 to make flattened surfaces 1-5 only in the vicinity of the branch holes. Thus compressive residual stresses are generated at inner peripheral edges of lower ends of the branch holes 1-2 to increase inner pressure strength at the inner peripheral edges of the lower ends of the branch holes. The autofrettage processing then is applied to generate compressive residual stresses on the whole inner peripheral surface of the main pipe rail 1 and all of the branch holes 1-2, thereby increasing the fatigue strength for inner pressures.

The flattened surfaces 1-5 projecting only neighborhoods of the branch holes 1-2 in the common rail shown in FIGS. 3 and 4 can be made using a pressing system with, for example, punches, rods, or the like to press a thick-walled portion of a main pipe rail 1 diametrically inward to project the flow passage 1-1 side so as to make the same substantially circular and at least flat. The surface caused by the projecting method to project inward, includes not only a flat surface, but also various curved configurations, such as arcuate surfaces, elliptical-shaped surfaces, or the like, and spherical configurations.

The common rail shown in FIG. 5 has stress concentration generated at an inner peripheral edge of a lower end of a branch hole decreased and fatigue strength for inner pressures increased by inserting a branch pipe 2 (or a branch joint) deeply into a branch hole 1-2 and projecting a tip end of the branch pipe 2 into a flow passage 1-1 from an inner peripheral surface of a main pipe rail 1. The branch pipe 2 is fixed by brazing. The autofrettage processing then is applied to generate compressive residual stresses on the whole inner peripheral surface of the main pipe rail 1 and the whole branch hole 1-2, thereby increasing the fatigue strength for inner pressures.

The common rail shown in FIG. 6 has flattened surfaces 1-5 provided at ends of branch holes 1-2 opened to a flow passage of a main pipe rail 1. Branch pipes 2 (or branch joints) are projected into the flow passage 1-1 from the flattened surfaces 1-5 and are subjected to brazing, whereby stress concentration generated at inner peripheral edges of lower ends of the branch holes 1-2 is decreased to increase inner pressure strength. The autofrettage processing then is applied in the same manner described above to generate compressive residual stresses on the whole inner peripheral surface of the main pipe rail 1 and all of the branch holes 1-2, thereby increasing the fatigue strength for inner pressures.

TABLE 2 indicates results of an endurance test carried Out by the use of common rails (steel type: S45C) shown in TABLE 1. In the present embodiment, the endurance test was carried out, in which test pressures composed of a base pressure 18 Mpa and peak pressures 140 to 230 Mpa were applied to respective common rails.

In addition, FIG. 8 shows a common rail, which is the same kind as those shown in FIGS. 1 and 2 and in which a cross section of a flow passage 1-1 of a main pipe rail 1 assumes a perfect circle. FIG. 9 shows a common rail, which is the same kind as those shown in FIGS. 3 and 4 and in which a cross section of a flow passage 1-1 of a main pipe rail 1 assumes a perfect circle in the same manner as the common rail shown in FIG. 8. FIG. 10 shows a common rail, which corresponds to the common rail shown in FIG. 5 and in which brazing is performed without having a tip end of a branch pipe 2 projected into a flow passage 1-1 of a main pipe rail 1. FIG. 11 shows a common rail, which corresponds to the common rail shown in FIG. 6 and in which brazing is performed without having a tip end of a branch pipe 2 projected from a flattened surface 1-5, which is provided at an end of a branch hole 1-2 opened to a flow passage 1-1 of a main pipe rail 1.

It is found from the results in TABLE 2 that all the common rails according to the invention exhibit an excellent fatigue strength for inner pressures. TABLE 1 SIZE TYPE MAIN PIPE RAIL BRANCH HOLE A (FIG. 1, DIAMETER 24 mm DIAMETER 3 mm FIG. 8) INSIDE DIAMETER 10 m B (FIG. 3, DIAMETER 24 mm DIAMETER 3 mm FIG. 9) INSIDE DIAMETER 10 m C (FIG. 5, DIAMETER 24 mm DIAMETER 3 mm FIG. 10) INSIDE DIAMETER 10 m D (FIG. 6, DIAMETER 24 mm DIAMETER 3 mm FIG. 11) INSIDE DIAMETER 10 m

TABLE 2 Yes or no of relaxation processing of Pressure Material stress Autogrettage fatigue Results Type for test concentration pressure pressure of test A Invention 1 Yes 350 18 to 230 10⁷ (FIG. 1) Pass Comparative No 350 18 to 160 Rupture example 1 (FIG. 8) Comparative No NO 18 to 160 Rupture example 2 (FIG. 8) B Invention 2 Yes 350 18 to 230 10⁷ (FIG. 3) Pass Comparative No 350 18 to 160 Rupture example 3 (FIG. 9) Comparative No NO 18 to 160 Rupture example 4 (FIG. 9) C Invention 3 Yes 300 18 to 190 10⁷ (FIG. 5) Pass Comparative Yes NO 18 to 190 Rupture example 5 (FIG. 5) Comparative No NO 18 to 140 Rupture example 6 (FIG. 10) D Invention 4 Yes 300 18 to 200 10⁷ (FIG. 6) Pass Comparative Yes NO 18 to 190 Rupture example 7 (FIG. 6) Comparative Yes NO 18 to 150 Rupture example 8 (FIG. 11)

As described above, a common rail for diesel engines, according to the invention, demonstrates remarkable effects of excellent durability for a flow passage in a main pipe rail and portions of a branch holes and increased fatigue strength for inner pressures at inner peripheral edges of lower ends of the branch holes. These effects are achieved because stress concentration at peripheral edges of the branch holes is decreased and autofrettage is then applied. As a result, high compressive residual stresses are achieved in the entire common rail due to the synergistic effect of relaxation of stress concentration applied at the peripheral edges of the branch holes and autofrettage applied on the whole flow passage in the main pipe rail and flow passages in the branch holes. 

1. A method for manufacturing a common rail for diesel engines, comprising: providing a main pipe rail having therein a flow passage extending in an axial direction, branch holes extending angularly to the axial direction and communicating with the flow passage; decreasing stress concentration in the vicinity of inner peripheral edges of openings of the branch holes; and subjecting a whole inner peripheral surface of the main pipe rail and the branch holes to autofrettage processing for generating a compressive residual stress on the entire inner peripheral surface of the main pipe rail and the branch holes, thereby increasing fatigue strength for pressure within the common rail.
 2. The method of claim 1, wherein the main pipe rail comprises a forging made of a material S45C and having a thick-walled tubular portion.
 3. The method of claim 1, further comprising forming flattened surfaces at least at the inner peripheral edges of openings of the branch.
 4. The method of claim 1, wherein the step of providing a main pipe rail comprises providing a main pipe rail with boss portions integral with the main pipe rail, the branch holes being in the boss portions and having a predetermined diameter.
 5. A method for manufacturing a common rail for diesel engines comprising: providing a main pipe rail having therein a flow passage extending in an axial direction, branch holes extending angularly to the axial direction and communicating with the flow passage; flattening at least areas of the flow passage adjacent the branch holes to decrease stress concentration; and subjecting a whole inner peripheral surface of the main pipe rail and the branch holes to autofrettage processing for generating a compressive residual stress on the entire inner peripheral surface of the main pipe rail and the branch holes, thereby increasing fatigue strength for pressure within the common rail.
 6. The method of claim 5, wherein the main pipe rail comprises a forging made of a material S45C and having a thick-walled tubular portion.
 7. The method of claim 5, further comprising forming flattened surfaces at least at the inner peripheral edges of openings of the branch.
 8. The method of claim 5, wherein the step of providing a main pipe rail comprises providing a main pipe rail with boss portions integral with the main pipe rail, the branch holes being in the boss portions and having a predetermined diameter.
 9. A method for manufacturing a common rail for diesel engines, comprising: providing a main pipe rail having therein a flow passage extending in an axial direction, branch holes extending angularly to the axial direction and communicating with the flow passage; applying pressing forces to areas of the main pipe rail substantially adjacent the branch holes to decrease stress concentration; and subjecting a whole inner peripheral surface of the main pipe rail and the branch holes to autofrettage processing for generating a compressive residual stress on the entire inner peripheral surface of the main pipe rail and the branch holes, thereby increasing fatigue strength for pressure within the common rail.
 10. The method of claim 9, wherein the main pipe rail comprises a forging made of a material S45C and having a thick-walled tubular portion.
 11. The method of claim 9, further comprising forming flattened surfaces at least at the inner peripheral edges of openings of the branch.
 12. The method of claim 9, wherein the step of providing a main pipe rail comprises providing a main pipe rail with boss portions integral with the main pipe rail, the branch holes being in the boss portions and having a predetermined diameter.
 13. A method for manufacturing a common rail for diesel engines, comprising: providing a main pipe rail having therein a flow passage in an axial direction and branch holes extending through the main pipe rail and communicating with the flow passage; inserting tip ends of branch pipes deeply into the branch holes and beyond an inner peripheral surface of the main pipe rail up to an interior of the flow passage for relaxing stress concentration in the vicinity of the branch holes; firmly connecting the branch pipes to the main pipe rail; and subjecting a whole inner peripheral surface of the main pipe rail and interiors of connections of the branch holes to autofrettage processing.
 14. The method of claim 13, wherein the main pipe rail comprises a forging made of a material S45C and having a thick-walled tubular portion.
 15. The method of claim 13, further comprising forming flattened surfaces at least at the inner peripheral edges of openings of the branch.
 16. The method of claim 13, wherein the step of providing a main pipe rail comprises providing a main pipe rail with boss portions integral with the main pipe rail, the branch holes being in the boss portions and having a predetermined diameter. 